KR101370012B1 - Quasi-phase matched device for speckle noise reduction with broadband green light, efficiency detecting apparatus for and dispaly device - Google Patents

Abstract

A quasi-phase matching optical element according to an embodiment of the present invention includes: a polarization reversal field; and a plurality of polarization reversal portions formed with the polarization reversal field adjacent to each other, wherein the polarization reversal portions are formed in an irregular QPM structure to be able to increase the linewidth itself of temperature when changing a wavelength, and to obtain a very high efficiency by reducing a speckle noise.

Description

Quasi-phase matched device for speckle noise reduction with broadband green light, Efficiency detecting apparatus for and Dispaly device }

The present invention relates to a quasi-phase matched optical device for a wide line width green light source with low speckle noise, and an optical conversion performance detecting device and a display device using the same, and more particularly, to a QPM (Quasi-Phase) having an irregular polarization inversion ratio. A quasi-phase matched optical element for a green light source having a matching structure, and an optical conversion performance detecting device and a display device using the same.

Display using laser light source has attracted attention as the definitive edition of next generation display, but the coherence of laser light source is so excellent that the mutual cancellation of light reflected from the screen surface during projection compared with lamp or LED light source- The constructive interference causes continuous light and dark patterns on the screen.

This phenomenon is called speckle noise, which is an unavoidable side effect when using a narrow line laser, which causes fatigue in the eyes of an audiovisual viewer. In order to reduce speckle noise, the most commonly used method is to insert DOE (Diffractive Optical Element) to artificially break the interference of light, to continuously rotate and change the polarization of light, and to change the laser wavelength slightly different from each other. How to use is used.

The method of artificially breaking the interference of light by inserting the above-described DOE or mechanically rotating the polarization of the light not only enlarges the system but also causes very large light loss.

In addition, a method using a quasi-phase matching (QPM) element is also used as an element for converting optical frequencies. In general, QPM devices are formed by photolithography using a mask on ferroelectric substrates such as LiNbO ₃ (LN), LiTaO ₃ (LT), and KTiPO ₄ (KTP) to form electrode patterns at intervals of several μm to several tens of μm. Then, it is produced by applying an electric field to reverse polarization.

An object of the present invention is to provide a quasi-phase matched optical device for a green light source having a very wide line width by extending the bandwidth by appropriately changing the period and polling ratio of the quasi-phase matched device (chirp and apodization), and a display device using the same.

The quasi-phase matched optical device according to an embodiment of the present invention includes a plurality of polarization inversion regions and a plurality of polarization inversion portions formed by neighboring polarization non-inversion regions, and the plurality of polarization inversion portions have an irregular QPM structure. do.

At least two or more of the plurality of polarization inversion units may have different polarization inversion ratios from each other.

At least two or more of the plurality of polarization inversion units may have different polarization inversion periods.

At least two or more of the plurality of polarization inversion units may have different polarization inversion ratios and different polarization inversion periods.

The polarization inversion unit is characterized in that the groups having a different polarization inversion ratio adjacent to each other to form a group.

The polarization inverting unit may be configured to have a group adjacent to each other having different polarization inversion periods different from each other.

The polarization inverting unit may form groups adjacent to ones having different polarization inversion periods different from each other, and groups having different polarization inversion ratios adjacent to each other form a group.

The polarization inversion unit is characterized in that the polarization inversion ratio is gradually increased in accordance with the traveling direction of the light.

The polarization inversion unit is characterized in that the polarization inversion ratio gradually decreases according to the traveling direction of the light.

The light source converting apparatus according to another exemplary embodiment of the present invention has a QPM (Quasi-Phase Matching) structure in which the polarization inversion period is aperiodic and the polarization inversion ratio is irregular to reduce speckle noise.

The light source conversion device is characterized by having a QPM structure of at least two groups of the same polarization inversion period.

The light source conversion device is characterized by having a QPM structure of at least two groups having the same polarization inversion ratio.

The light source converting device is characterized in that it has a QPM structure in which the polarization reversal ratio is increased according to the traveling direction of light.

The light source conversion device is characterized in that it has a QPM structure in which the polarization inversion ratio decreases according to the traveling direction of light.

In accordance with still another aspect of the present invention, an optical conversion performance detection device includes: a light source generating and outputting light; A light intensity controller for adjusting the intensity of the output light; A light focusing unit configured to change a path of light output from the first optical element and to focus incident light; A light conversion unit converting the polarization inversion period aperiodically to the incident light and irregularly changing the polarization inversion ratio; An optical separator for separating the incident light having a different wavelength from each other and green light generated by a second harmonic generation (SHG) process; And a detector for detecting the intensity of the separated green light.

The optical conversion performance detection device further comprises a beam blocker for a safety device that blocks the light separated from the optical splitter.

The light intensity controller includes a λ / 2 polarizer for rotating the polarization of the fundamental wave to 90 °, and a polarizer for maintaining the 90 ° rotated polarization.

The light converging unit may include first and second mirrors and first lenses for freely changing the path of the incident light and directing them to a desired location, and first and second lenses for focusing the incident light on a sample.

The light conversion unit may include any one of the above-described quasi-phase matching optical devices.

Laser light source display device according to another embodiment of the present invention is characterized in that it comprises any one of the above-described light source conversion device.

The quasi-phase matched optical device for a green light source having a wide line width according to embodiments of the present invention and a display device using the same have the effect of widening the temperature and the wavelength line width itself during wavelength conversion.

In addition, the quasi-phase matching optical device for a green light source having a wide line width according to embodiments of the present invention and a display device using the same have an effect of obtaining very high efficiency by reducing speckle noise.

1 is a block diagram of a light conversion performance detection device for a green light source according to an embodiment of the present invention;
FIG. 2 is a conceptual diagram illustrating sign inversion of a nonlinear coefficient according to a variable polarization inversion period applied to the light conversion performance detecting apparatus of FIG. 1 according to a traveling direction of light.
3 is a view illustrating a polarization structure of a quasi-phase matched optical device for a green light source having an irregular polarization inversion period and a polarization inversion ratio according to an embodiment of the present invention.
4 (a) and 4 (b) are structural diagrams having irregular polarization inversion periods and polarization inversion ratios of gradually increasing / decreasing the quasi-phase matching optical device for the green light source according to an embodiment of the present invention.
5 (a) to (d) are micrographs of optical devices manufactured by the drawings;
6 is a result of wavelength scanning at a specific temperature,
7 is a result of temperature scanning at a specific wavelength,
8 is a view for comparing the relative wavelength line width in the length of the same sample,
9 is a view for comparing the relative temperature line width in the length of the same sample.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to designate the same or similar components throughout the drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. The terms used in the singular may also include a plurality of concepts . In addition, the preferred embodiments of the present invention will be described below, but it is needless to say that the technical idea of the present invention is not limited thereto and can be variously modified by those skilled in the art.

FIG. 1 is a block diagram of an optical conversion performance detecting device for a green light source according to an embodiment of the present invention, and FIG. 2 is a sign inversion of a nonlinear coefficient according to a variable polarization inversion period applied to the optical conversion performance detecting device of FIG. Is a conceptual diagram expressed according to the traveling direction of light.

3 is a diagram illustrating a polarization structure of a quasi-phase matched optical device for a green light source having an irregular polarization inversion period and a polarization inversion ratio according to an embodiment of the present invention. In FIG. 3, the black block is a portion having a basic polarization, and the white block is a portion showing a region in which polarization is reversed.

4 (a) and 4 (b) each show a structure having an irregular polarization inversion period and a polarization inversion ratio in which the phase-matching optical device for a green light source according to an embodiment of the present invention gradually increases / decreases. .

Referring to FIG. 1, an apparatus 100 for detecting light conversion performance according to an exemplary embodiment of the present invention may include a light source 110 generating and outputting light; Light intensity control unit 120 for adjusting the intensity of the output light; A light focusing unit 130 for changing a path of light output from the light intensity adjusting unit and focusing incident light; A light conversion unit 140 converting the polarization inversion period aperiodically to the incident light and irregularly changing the polarization inversion ratio; An optical separator 150 for separating the incident light having a different wavelength from each other and green light generated by a second harmonic generation (SHG) process; And a detector 160 for detecting the intensity of the separated green light.

The optical conversion performance detection device 100 preferably further includes a beam blocker 170 for a safety device that blocks the light separated from the optical separator.

The light sources Nd: YVO ₄ and 110 have a wavelength of 1064 nm, a repetition rate is 10 kHz, and a pulse duration is 38 ns.

The light intensity controller 120 includes a λ / 2 polarizer 121 for rotating the polarized light of the fundamental wave by 90 degrees, and a polarizer 122 for maintaining the polarized light rotated by 90 degrees. The first and second mirrors 131 and 132 for freely changing the path of the incident light and directing it to a desired location, and the light output from the first lens 133 and the light changing unit 140 that focus the incident light on a sample. The second lens 134 focuses on the optical separator 150. Here, the focal length f of the first lens 133 is 200 mm, and the focal length f of the second lens 134 is 750 mm.

The optical separator 150 is an optical device that separates 1064 nm, which is an input wave having two different wavelengths, and 532 nm, which is green light generated by a second harmonic generation (SHG) process. That is, for example, 1064 nm input wave reflects 99% and 532 nm green light transmits 95%.

As shown in FIG. 2, the light conversion unit 140 has an irregular Tandem-QPM device structure that follows the light conversion principle according to the variable polarization inversion period. This may be represented by Equation 1.

Here, B (z) means the electric field strength of the input wave in the traveling direction z in the medium. A (z) denotes the electric field strength of the second harmonic wave generated according to the traveling direction, and k ₁ and k ₂ refer to the wavefront factor of the input wave and the generated second harmonic wave, respectively. d _eff is the effective nonlinear coefficient of the medium, c is the luminous flux, ω ₁ and ω ₂ are the frequencies between the input and secondary harmonic waves, respectively, and Δk is the wave mismatch between the input and generation waves.

3 to 4, in the quasi-phase matched optical device 200 according to another exemplary embodiment, a polarization inversion part in which a polarization inversion region 210 and a polarization non-inversion region 220 are formed adjacent to each other. A plurality of 230 is included. The plurality of polarization inversion units 230 have an irregular QPM structure, at least two or more of the plurality of polarization inversion units 200 have different polarization inversion ratios, and at least two or more have different polarization inversion periods. It is possible.

Here, the gray portion is the polarization inversion region 210, that is, the polarization inversion is performed, and the white portion is the polarization non-inversion region 210, that is, the polarization inversion is not performed.

The plurality of polarization inversion units 200 may be formed in a structure in which groups having different polarization inversion ratios are adjacent to each other, or in which groups having different polarization inversion periods are mutually adjacent to form a group.

The polarization inversion unit 200 may be formed in a structure in which the polarization inversion ratio is gradually increased according to the traveling direction of the light, or the polarization inversion ratio is gradually decreased in the traveling direction of the light.

More specifically, the new method in one embodiment of the present invention provides a green light source having a very wide line width by extending the bandwidth causing wavelength conversion by appropriately changing the period and polling ratio of the phase matching device. It is a method of making Tandem-QPM devices to be developed (formerly 0.1 nm, 3 nm in the present invention). In addition, this Tandem-QPM device allows for 30 times wider line width than conventional uniform devices, and the speckle noise reduction effect is 10 times higher than that of conventional quasi-phase matching devices or wavelength conversion devices using birefringence. Bring it.

That is, the present inventors have developed an alternative light source having a wide line width (3 nm or more) by solving the problem of speckle noise generated when a conventional line light source having a narrow line width (0.1 nm) is used as a display light source.

Speckle noise is due to the texture of the laser, and when the display is implemented using the laser, light reflected from the surface of the screen causes mutual reinforcement or destructive interference, which causes light intensity to be modulated, resulting in bright and dark patterns on the surface of the screen. It appears constantly and these bright and dark spots appear on the whole screen irregularly, which causes the fatigue of the human eye. This is one of the biggest issues that must be solved when using a laser light source as a display light source.

In the exemplary embodiment of the present invention, an efficient green light source is used for a display by using a second harmonic wave (SHG) process using a conventional quasi-phase matching device, but such a light source has a problem that it is difficult to overcome the wavelength line width of 0.1 nm or less. It is a structure that breaks the phase-matching structure that is periodically polarized inverted, and creates a phase-matched structure with a non-periodic and irregular polarization inversion ratio, through which the line width is very wide (more than 20 times) green. The light source was developed.

Since the most important point of the invention is the combination of aperiodic and irregular polarization inversion ratios, the creation of mask design based on theoretical calculations accounts for more than 60%.

Next, 20% of the mask was created by pattern lithography through photolithography, and when the pattern was successful, the electrode was completed. negative) Performing polarization reversal using the multi-pulse polling method accounts for about 20% of the weight, and device fabrication is completed.

In aperiodic, aperiodic / apodized structures, the invention of a structure with a temperature line width of 100 degrees or more and a wavelength line width of 19 nm or more is the key. By using pulses, we create a quasi-phase matching device that follows the original mask design form.

FIG. 3 shows an aperiodic structure with a wide bandwidth line width with varying phase-matching conditions, and FIG. 4 shows a stable apodization with varying duty ratios. Here, duty ratio = Reversed polarization / Period.

In other words, the effective nonlinear coefficient (d _eff ), which is the most important in the existing conventional periodic phase-matching structure, which is one of the nonlinear wavelength conversion methods, changes only the sign at the time of polarization reversal, and its value is constant. It is determined from the relation with the coherence length (l) of the harmonic wave.

However, since this conventional method is capable of only narrow linewidth wavelength conversion, the present invention proposes a new method for enabling wide linewidth wavelength conversion, as shown in FIGS. 3 to 4.

Referring to FIGS. 2 and 3, unlike the conventional technique, the process of performing chirp while gradually decreasing and increasing the cycle out of the periodic structure is performed such that the cycle itself is distributed in an array using permutations. It is expressed as Equation 2, which is an aperiodic structure.

Λ _p means the period of polarization inversion up to the pth. ΔΛ means a difference value between two arbitrarily selected polarization inversion periods, and in particular ΔΛ _i means a difference value between the i th and i + 1 th polarization inversion periods. L means the length of the entire device polarized inverted to the nth, and a means the duty ratio. i has an integer value along with n and p, and the maximum value i can have becomes p.

Next, while the period of apodization is constant (here, the period is called a period in which the existing polarization is maintained and the polarization inversion is added), the ratio of the polarization inversion of the period is determined. The duty ratio is defined as a duty ratio, and the duty ratio is changed to produce a change.

Li 'and Li "refer to the total length of the medium up to the nth period of the region where the period is constant and the duty ratio is different (that is, the apodized part), where Li' is constant at Λ 'and Li" is Λ I is constant. I represents an integer sequence from 1 to p '. I' and i have arbitrary integers like i.

Therefore, in the present invention, in designing an optical device module capable of generating a green light source having a wide line width structure according to the above-described QPM structure, by combining two technologies simultaneously such as aperioc and apodization, The device was completed.

In detail, as shown in FIGS. 3 and 4, the polarization inversion is to be performed in the direction in which the laser beam of the light guiding element travels. The polarization inversion is largely divided into three regions, for example, 6.6 μm to 6.8 to generate green light. For the purpose of distributing the period up to μm, the period of polarization reversal is gradually increased in the first area, fixed period in the middle, and aperiodicly arranged in the direction of decreasing the period in the last area. It is proposed to finely enter each of the true three parts so that the duty ratio, which is the polarization reversal ratio, has an arrangement ranging from about 0.2 to 0.5 based on 0.5.

The quasi-phase matched optical device according to an embodiment of the present invention comprises the steps of making a mask according to the aperture width (a) in the mask and the aperture ratio (R = a / Λ) determined by the QPM period (Λ), the first order Positive photoresist coating under conditions (900 rpm, 10 seconds) and secondary conditions (1500 rpm, 60 seconds), soft baking for 100 ° C.-60 minutes, 10 seconds at UV power of 15.5-16 mW / cm ² UV exposure, developing for 40 seconds, hard baking under the conditions of 80 ° C./30 min, 100 ° C./30 min, 120 ° C./30 min, 140 ° C./30 min, 160 ° C./30 min, 180 ° C./30 min. And dicing to form a chip shape, electric field polling using a liquid electrode with electric pulses, polishing with 3 minutes lapping and 60 minutes polishing.

Comparison factor PPLN APPLN Comparison result Δλ 0.38nm ~ 9nm 23.7 times bigger ΔT 4.9 ℃ 110 ° C 22.4 times bigger d _eff 21 pm/V ~ 4.9pm / V 4 times smaller

Thus, as shown in Table 1, speckle contrast can be reduced to almost 1/5. Speckle noise is inversely proportional to the square root of the laser line width. If the speckle contrast (C) is less than 4%, the human eye cannot distinguish noise. The definition formula of speckle contrast is shown in Equation 4.

Where C is speckle contrast, V is the center frequency of light, λ is the wavelength, σ _h is the surface roughness of the screen, and each of θ _i and θ _o is the angle of incidence and observation.

5 (a) to 5 (d) are micrographs of optical devices manufactured by the drawings. In FIG. 5, the optical device is Z-cut CLN having a thickness of 0.5 nm. The interaction length is 5.5 mm, the channel width is 300 μm and 350 μm, and the sample size is 5.5 mm × 3.5 mm.

6 shows wavelength scanning results when a specific temperature is 61 ° C. 7 shows the results of temperature scanning when a specific wavelength is 1.064 μm.

8 is a view for comparing the relative wavelength line width in the length of the same sample. In Figure 8 (a) is the theoretical wavelength scan results through the structure proposed by the present invention using the LN having a length of 5.5mm, (b) is a conventional periodic polarization inverted structure.

9 is a view for comparing the relative temperature line width in the length of the same sample. In Figure 9 (a) is the theoretical temperature scan results through the structure proposed by the present invention using the LN having a length of 5.5mm, (b) is a conventional periodic polarization inverted structure.

PPLN APPLN ratio Peak value 4.07 × 10 ^-6 1.94 × 10 ^-6 21 wavelength
FWHM 0.38nm 8.8nm 23.2 Temperature
FWHM 4.9 ℃ ~ 110 ℃ 22.4

[5.5mm, r = 20/21, R = 0.3 ~ 0.5]

Table 2 shows the peaks that can compare the efficiency of using LN crystals with a conventional periodic polarization inversion structure at the same device length of 5.5 mm and when generating green light, a second harmonic wave in the apodized structure of the present invention. It is a table | surface which showed the value, FWHM which is half wavelength width, and FWHM which is half temperature width.

In the above description, 5.5mm corresponds to the length of the device, and 'r = 20/21' means that 20 of the structures divided into 21 areas are apodized. That is, one area is an aperiodic area. 'R = 0.3 ~ 0.5' means to change the duty ratio from 0.3 to 0.5.

It will be apparent to those skilled in the art that various modifications, substitutions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. will be. Accordingly, the embodiments disclosed in the present invention and the accompanying drawings are not intended to limit the technical spirit of the present invention but to describe the present invention, and the scope of the technical idea of the present invention is not limited by the embodiments and the accompanying drawings. . The protection scope of the present invention should be interpreted by the following claims, and all technical ideas within the equivalent scope should be interpreted as being included in the scope of the present invention. In addition, claims that do not have an explicit citation in the claims may be combined to form an embodiment or included in a new claim by amendment after the application.

110: light source 120: light intensity control unit
121: lambda / 2 polarizing plate 122: polarizer
130: light focusing unit 131, 132: first and second mirrors
133, 134: first and second lenses 140: light conversion unit
150: optical separator 160: detector
170: beam blocker 200: quasi-phase matched optical element
210: polarization inversion area 220: polarization non-inversion area
230: polarization inversion

Claims (20)

A plurality of polarization inversion regions and a polarization inversion region, the polarization inversion region formed adjacent to each other,
The plurality of polarization inverting portion has an irregular QPM structure,
At least two or more of the plurality of polarization inversion units have different polarization inversion ratios and different polarization inversion periods,
The polarization inversion ratio is a ratio of the width of the polarization inversion region to the width of the entire region including the polarization inversion region and the polarization non-inversion region,
And the polarization inversion period is a total width obtained by adding up the width of the polarization inversion area and the width of the polarization non-inversion area. delete delete delete The method of claim 1,
And said polarization inverting portion adjacent to each other having a different polarization inversion ratio to form a group. The method of claim 1,
And said polarization inverting portion forms a group adjacent to each other having a different polarization inversion period from each other. The method of claim 1,
And the polarization inverting portion forms a group adjacent to each other having different polarization inversion periods different from each other, and forms a group adjacent to each other having a different polarization inversion ratio from each other. The method of claim 5, wherein
And the polarization inversion unit gradually increases the polarization inversion ratio according to the traveling direction of light. The method of claim 5, wherein
And wherein the polarization inversion unit gradually decreases the polarization inversion ratio in accordance with the traveling direction of the light. In the light source conversion device,
It has a quasi-phase matching structure with a non-periodic polarization inversion period and irregular polarization inversion ratio to reduce speckle noise.
The polarization inversion ratio is a ratio of the width of the polarization inversion region to the width of the entire region including the polarization inversion region and the polarization non-inversion region,
And the polarization inversion period is a total width obtained by adding up the width of the polarization inversion region and the width of the polarization non-inversion region. 11. The method of claim 10,
And the light source conversion device has a QPM structure of at least two groups of the same polarization inversion period. 11. The method of claim 10,
And the light source converting device has a QPM structure of at least two groups having the same polarization inversion ratio. 13. The method according to claim 11 or 12,
And the light source converting device has a QPM structure in which the polarization inversion ratio is increased in accordance with the traveling direction of light. 13. The method according to claim 11 or 12,
And the light source converting device has a QPM structure in which the polarization inversion ratio decreases according to the traveling direction of light. A light source for generating and outputting light;
A light intensity controller for adjusting the intensity of the output light;
A light focusing unit which changes a path of light output from the first optical element and focuses incident light;
A light conversion unit converting the polarization inversion period aperiodically to the incident light and irregularly changing the polarization inversion ratio;
An optical separator for separating the incident light having a different wavelength from each other and green light generated by a second harmonic generation (SHG) process; And
A detector for detecting the intensity of the separated green light
Including;
The polarization inversion ratio is a ratio of the width of the polarization inversion region to the width of the entire region including the polarization inversion region and the polarization non-inversion region,
And wherein the polarization inversion period is a total width obtained by adding up the width of the polarization inversion area and the width of the polarization non-inversion area. The method of claim 15,
The optical conversion performance detection device further comprises a beam blocker for a safety device for blocking the light separated by the optical splitter. The method of claim 15,
The light intensity control unit includes a λ / 2 polarizer for rotating the polarization of the fundamental wave 90 degrees, and a polarizer for maintaining the polarized light rotated 90 degrees. The method of claim 15,
And the light focusing unit includes first and second mirrors for freely changing a path of the incident light and directing the light to a desired location, and first and second lenses for focusing the incident light. The method of claim 15,
The optical conversion unit detecting the optical conversion performance, characterized in that it comprises a quasi-phase matching optical device according to claim 1. A laser light source display device comprising the light source conversion device according to claim 10.

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